1. Field of the Invention
The present invention relates to a driving circuit of an insulated gate device, and particularly to a driving circuit of an insulated gate device that prevents faulty turning-on of the device and carries out a high-speed turning-off operation.
2. Background Art
FIG. 11 is a circuit diagram showing a configuration of a related driving circuit of an insulated gate device. As is shown in FIG. 11, one end of a load 102, such as a resistive load or an inductive load, is connected to a power supply 101 and the other end of the load 102 is connected to a load driving control device (high functional Metal Oxide Semiconductor Field Effect Transistor (MOSFET)) 103. The load driving control device 103 is provided with three terminals of a drain terminal 104, a gate terminal 105 and a source terminal 106. The drain terminal 104 is connected to the other end of the load 102 and the source terminal 106 is connected to the ground 107. In addition, to the gate terminal 105, a gate signal is externally inputted. The load driving control device 103 is formed of a driving circuit unit 117 and a power unit 118, which are formed in one semiconductor chip.
The power unit 118 includes a power MOSFET 108 which is an insulated gate device with its turning-on and turning-off controlled by the driving circuit unit 117.
Moreover, the load driving control device 103 has a gate potential line 123 and a ground potential (source potential) line 124. The gate potential line 123 connects the gate terminal 105 and the gate of the power MOSFET 108 through a gate resistor 113. Between the gate potential line 123 and the ground potential (source potential) line 124, there are provided a temperature detecting sensor 111 that detects a temperature, a logic circuit 112 that carries out signal processing of the signal of the temperature detecting sensor 111 and determines a threshold voltage of the load driving control device 103 and a gate voltage controlling MOSFET 114 that controls the shut down the voltage of the gate potential line 123.
Moreover, the drain terminal 104 and the drain of the power MOSFET 108 are connected by a drain potential line 122. Between the drain potential line 122 and the ground potential (source potential) line 124, a current detecting sensor 110 is provided and between the gate potential line 123 and the ground potential (source potential) line 124, a gate voltage control circuit 115 is also provided that controls the voltage level of the gate potential line 123 by receiving a signal of the current detecting sensor 110.
Besides this, between the gate potential line 123 and the ground potential (source potential) line 124, a diode 109 and the gate resistor 113 are provided as protecting devices of the power MOSFET 108. The connection point of the diode 109 and the gate resistor 113 is connected to the gate terminal 105. Furthermore, a constant current source 116 is provided for pulling-down the voltage of the gate potential line 123 so as not to cause the power MOSFET 108 to turn-on even though a noise is inputted to the gate terminal 105.
The load driving control device 103 functions as a switching device for driving the load 102. Moreover, the load driving control device 103, in addition to the switching function, has an overcurrent detecting function for preventing the load driving control device 103 itself from being broken down by a large current flowing in the load driving control device 103 in case such as short circuit of the load 102, an overheating detecting function for preventing the load driving control device 103 itself from being broken down by heat generated due to the large current and a gate protecting function of the switching device. The overcurrent detecting function and the overheating detecting function are operated with the gate voltage used as a power supply voltage.
The overheating detecting function is operated as follows. Namely, with an increase in temperature, when the voltage of the output line 121 of the temperature detecting sensor 111 (the input line to the logic circuit 112) reaches a specified voltage, the logic circuit 112 applies a voltage, which brings a gate voltage controlling MOSFET 114 into a turned-on state, to a gate 119 of the gate voltage controlling MOSFET 114. This makes the voltage of the gate potential line 123 lower than the threshold voltage of the power MOSFET 108 to turn-off the power MOSFET 108 to turn-off the load driving control device 103.
The overcurrent detecting function is operated as follows. Namely, when a voltage of an input line 120 provided from the current detecting sensor 110 to the gate voltage control circuit 115 reaches a specified voltage with an increase in a current flowing between the drain terminal 104 and the ground terminal (source terminal) 106, the gate voltage control circuit 115 restricts a current flowing between the drain terminal 104 and the ground terminal (source terminal) 106 by reducing the voltage of the gate potential line 123.
Moreover, a threshold voltage determination function is also provided by the logic circuit 112 and the gate voltage controlling MOSFET 114. The threshold voltage determination function is a function by which until the threshold voltage of the load driving control device 103 is applied to the gate terminal 105, the gate voltage of the power MOSFET 108 provided by the gate potential line 123 is made to be lower than the threshold voltage of the power MOSFET 108 so that the power MOSFET 108 is not made turned-on.
FIG. 12 is a timing chart showing a threshold voltage determination function. Here, FIG. 12 shows voltage waveforms with respect to a gate voltage Vin of the gate terminal 105, a gate voltage Vg of the power MOSFET 108 (a voltage of the gate potential line 123), a drain voltage Vd of the power MOSFET 108 (a voltage of the drain potential line 122), and a gate voltage Va of the gate voltage controlling MOSFET 114 when a triangular wave voltage is inputted to the gate terminal 105. As shown in FIG. 12, by carrying out the on-off control of the gate voltage controlling MOSFET 114, until the voltage Vin of the gate terminal 105 reaches the threshold voltage VIN(th) of the load driving control device 103, the gate voltage Vg of the power MOSFET 108 is made to be lower than the threshold voltage Vg(th) of the power MOSFET 108. By making the threshold voltage Va(th) of the gate voltage controlling MOSFET 114 and the threshold voltage Vg(th) of the power MOSFET 108 as Va(th)<Vg(th) with variations occurred in the manufacturing process included, the gate voltage Vg of the power MOSFET 108 can be controlled in this way by the driving circuit unit 117 to enable the determination of the threshold voltage VIN(th) of the load driving control device 103.
Incidentally, between the gate and the drain of the power MOSFET 108, a relatively large parasitic capacitor Cgd (not shown) is formed.
Therefore, when switching the state of the power MOSFET 108 from a turned-on state to a turned-off state, the gate voltage Vg is raised by the charging current of the parasitic capacitor Cgd. At this time, with the voltage Vin at the gate terminal 105 being lower than the threshold voltage of the gate voltage controlling MOSFET 114, the gate voltage controlling MOSFET 114 is made turned-off and no charging current is pulled-out by the gate voltage controlling MOSFET 114 to be such a problem as to lengthen the turning-off time.
Thus, for quickly pulling-out the charging current, a measure is generally taken in which a resistor or a constant current source is disposed between the gate of the power MOSFET 108 and the ground potential line 124 or between the gate terminal 105 and the ground potential line 124 to thereby minimize the impedance between the gate of the power MOSFET 108 and the ground potential line 124.
Moreover, as a technique that effectively reduces a surge voltage generated when a power MOSFET is made turned-off and a turn-off loss at this time, there is the technique described in JP-A-2008-675934. According to the technique, a current source circuit and a current adjusting circuit are provided. The current source circuit is a circuit that discharges a gate capacitor when turning-off a current flowing in a main terminal of a power MOSFET and the current adjusting circuit is a circuit that adjusts a value of a current flowing when a gate capacitor is discharged through the current source circuit.
In the technology described in JP-A-2008-675934, the driving circuit is formed so that, when the power MOSFET is turned-off by a gate signal, the current source circuit is connected and, when the power MOSFET is in a state of being turned-on, the current source circuit is disconnected. Here, the output current of the current source circuit is made variable and by the current adjusting circuit, the output current of the current source circuit is made to have a constant value until the voltage across the main terminals of the power MOSFET begins to increase and, with an increase in the voltage across the main terminals, the output current of the current source circuit is made gradually decreased.
As was explained in the foregoing, between the gate and the drain of the power MOSFET 108, the relatively large parasitic capacitor Cgd is formed. This causes, when the voltage of the power supply 101 is promptly increased with the power MOSFET 108 being in a turned-off state, a current Igd charging the parasitic capacitor Cgd to flow. The current Igd flows to the ground 107 through the constant current source 116 and the gate resistor 113. The flow of the current Igd in the constant current source 116 and the gate resistor 113 causes the voltage of the gate potential line 123 to be raised and when the raised voltage exceeds the threshold voltage of the power MOSFET 108, the state of power MOSFET 108 is switched from a turned-off state to a turned-on state. Such a phenomenon remarkably appears particularly when the voltage at the gate terminal 105 is in a state of being lower than the threshold voltage of the gate voltage controlling MOSFET 114 due to the gate voltage controlling MOSFET 114 being in a turned-off state and carrying out no pulling-out of a current.
With the technique described in JP-A-2008-675934, however, no measure is taken against the case in which the power supply voltage is suddenly increased when the power MOSFET is in a turned-off state. Thus, under such a condition, the power MOSFET in a turned-off state is brought into a faulty turned-on state. For preventing such a problem, the value of the output current of the current source circuit must be always kept to be equal to or more than a certain constant current value.
In this case, however, the voltage applied to the gate terminal is pulled-down to cause such problems as a decrease in a conducting ability (increase in an on-resistance Ron) and an increase in consumed power of the power MOSFET due to a decrease in the gate voltage of the power MOSFET at normal turning-on.
Accordingly, an object of the invention is to provide a driving circuit of an insulated gate device which circuit can actualize prevention of faulty turning-on of the device and high speed turning-off of the device while reducing influences on a normal operation (consumed power and Ron).